1. Field of the Invention
The present invention relates to a multistage vacuum pump assembly.
2. Description of the Prior Art
JP 07(1995)-305689 shows a conventional multistage vacuum pump assembly. Each pump stage is a single pump which comprises a housing which comprises an inlet, an outlet, a pump room and a rotor unit rotatably located in the pump room. Rotation of the rotor unit causes a fluid to be moved from the inlet to the outlet. For example, air in the inlet is sucked to the outlet. Plural pump stages are fluidically connected to each other in series via at least one tube. An inlet of the pump assembly at the upstream end of the series is connected to a room to be evacuated and an outlet of the pump at the downstream end of the series is connected to the atmosphere.
Since the pump stages are connected in series and all have the same performance characteristics, the total vacuum performance of the multistage vacuum pump assembly can be increased only by increasing the rate of rotation of the rotor unit of the pump stages. However the maximum rate of rotation is limited, and an expected total vacuum performance might not be achievable. Even if the expected performance can be achieved by operating the rotor units at high speeds, the life of the multistage vacuum pump assembly might be shortened due to the high speed operation.